The present invention relates to a time base, i.e. a device comprising a resonator and an integrated electronic circuit for driving the resonator into oscillation and for producing, in response to this oscillation, a signal having a determined frequency.
Time bases, or frequency standards, are required in a large variety of electronic devices, ranging from wristwatches and other timepieces to complex telecommunication devices. Such time bases are typically formed by an oscillator including a quartz resonator and an electronic circuit for driving the resonator into oscillation. An additional division chain may be used to divide the frequency of the signal produced by the oscillator in order to obtain a lower frequency. Other parts of the circuit may serve to adjust the frequency, for example by adjusting the division ratio of the division chain. The components of the electronic circuit are advantageously integrated onto a single semiconductor substrate in CMOS technology. Other functions, not directly related to the frequency processing, may be integrated onto the same substrate.
Advantages of quartz resonators are their high quality factor Q leading to good frequency stability and low power consumption as well as their good temperature stability. A disadvantage of typical time bases using quartz resonators however resides in the fact that two components, namely the quartz resonator and the integrated electronic circuit, are required in order to provide a high-precision frequency. A discrete quartz resonator requires board space which is scarce in many cases. For instance, a standard quartz resonator for wristwatch applications requires space of the order of 2xc3x972xc3x976 mm3. Moreover, additional costs are caused by the assembly and connection of the two components. Yet, space and assembly costs are major issues, especially in the growing field of portable electronic devices.
It is thus a principal object of the present invention to provide a solution to the above-mentioned problems by providing a time base comprising an integrated resonator.
Another object of the present invention is to provide a time base that may be fully integrated on a single substrate, that is suitable for mass production and that is compatible with CMOS technology.
Still another object of the present invention is to provide a time base comprising a resonator having an improved quality factor Q and thereby a greater frequency stability and low power consumption.
Yet another object of the present invention is to provide such a time base which is low-priced and requires only a very small surface area on a semiconductor chip.
Accordingly, there is provided a time base comprising a resonator and an integrated electronic circuit for driving said resonator into oscillation and for producing, in response to said oscillation, a signal having a determined frequency, characterised in that said resonator is an integrated micromechanical ring resonator supported above a substrate and adapted to oscillate, according to a first oscillation mode, around an axis of rotation substantially perpendicular to said substrate, said ring resonator comprising:
a central post extending from said substrate along said axis of rotation;
a free-standing oscillating structure connected to said central post and including:
an outer ring coaxial with said axis of rotation; and
a plurality of spring elements disposed symmetrically around said central post and connecting said outer ring to said central post;
and
at least one pair of diametrically opposed electrode structures disposed around said outer ring and connected to said integrated electronic circuit.
An advantage of the time base according to the present invention lies in the fact that the micromechanical ring resonator exhibits a high quality factor Q. Quality factors as high as 2xc3x97105 have been measured. For comparison, tuning-fork quartz resonators usually exhibit values between 5xc3x97104 and 1xc3x97105 after laser trimming of the fork tines. Different design features favouring a high quality factor Q are the object of dependent claims and will be described hereinafter in detail.
In addition, for a given resonant frequency, the surface area required on the substrate to form the ring resonator is small in comparison with other resonators.
According to one aspect of the invention, the electronic circuit is advantageously integrated on the substrate together with the micromechanical ring resonator, thereby leading to a low-priced time base. A lower price is also obtained by wafer-level packaging of the resonator using wafer-bonding technology.
It must be pointed out that ring resonators having similar features are known from sensing devices, such as angular rate sensors, accelerometers or gyroscopes. For instance U.S. Pat. No. 5,450,751 to Putty et al. and U.S. Pat. No. 5,547,093 to Sparks both disclose a micromechanical ring resonator for a vibratory gyroscope comprising a plated metal ring and spring system supported above a silicon substrate. U.S. Pat. No. 5,872,313 to Zarabadi et al. discloses a variant of the above sensor which is configured to exhibit minimum sensitivity to temperature variation. U.S. Pat. No. 5,025,346 also discloses a ring resonator for use as a micro-sensor in a gyroscope or an angular rate sensor.
None of the above-cited documents however indicates or suggests using such a type of ring resonator in an oscillator circuit to act as a frequency standard or time base. Moreover, a number of design features (e.g. the shape and number of spring elements) of the ring resonators disclosed in these documents are such that they would not be suitable for horological applications where frequency stability and low power consumption are essential. For instance, the resonating structures disclosed in U.S. Pat. No. 5,025,346 exhibit a quality factor ranging from 20 to 140 which is too low for being used in a highly precise time base in horological applications, whereas quartz resonators used in horological applications exhibit quality factors of the order of 1xc3x97104 to 1xc3x97105.
According to the present invention, various design features are proposed which lead to a high quality factor Q, a high stability of the oscillation frequency against variations in the amplitude of the driving voltage, and tolerance of fabrication process variations. In fact, one of the major objectives for an application as an oscillator is a high quality factor Q. A high quality factor Q results in a stable oscillation with low phase noise and low power consumption, as is required for horological applications.
According to other aspects of the present invention, various mechanisms are proposed for substantially compensating for the effect of temperature on the resonant frequency of the ring resonator.
According to another aspect of the present invention, a temperature measuring circuit may further be integrated on the substrate in order to compensate for the effect of temperature on the frequency of the signal produced by the time base. Such compensation of the resonator""s temperature dependency may easily be effected since the ring resonator of the present invention has the advantage of exhibiting substantially linear temperature characteristics.
According to still another aspect of the present invention, a second micromechanical ring resonator may be formed on the substrate in order to allow temperature compensation. According to another aspect of the invention, temperature compensation is also achieved by using a single micromechanical ring resonator which is operated simultaneously with two oscillation modes having different resonant frequencies.